Photoshop for iPad error: could not use tool because target channel is hidden

I am getting an error message that says "Could not use the brush tool because the target channel is hidden" (or eraser, or whatever tool). It won't work on any of my layers. Sometimes I can get it to work momentarily but then it quickly reverts to the same message. I have closed and restarted Photoshop and my iPad, and installed the lastest update. I am pretty new to Photoshop for iPad, and I'm totally at a loss - I didn't think you could even do anything with channels on an iPad. My cat was sleeping on the keyboard before this started so I wonder if he accidentally pressed something. Please help!